1. Field of the Invention
The present invention relates to a method of laminating disc-shaped substrates, particularly to a method of integrally laminating two substrates using an adhesive sheet.
2. Prior Art
Computers, especially personal computers have become remarkably popular recently, and the kinds of storage media to be used thereby and the capacity of such storage media, particularly storage discs has increased. There are a magnetic disc, an optical disc (e.g., CD-ROM), an optical magnetic disc (e.g., MO), etc. as the storage disc. The demand for the optical discs as storage discs has increased recently.
Exemplifying an optical disc called a DVD, a disc-shaped substrate that is a single plate constituting the DVD is required to have a thickness of 0.6 mm and an outer diameter of 120 mm and an inner diameter of its central hole of 15 mm. Since such a thin disc-shaped substrate formed of a single plate is low in mechanical strength and is easily deformable, and in view of storage capacity, the disc-shaped substrates each having the same thickness (0.6 mm) are bonded to each other to form an integrated substrate in a practical use thereof.
Such a high density storage disc including a DVD is generally used as a laminated structure as set forth above, but not used as a single plate. In such a case, the upper and lower disc-shaped substrates must to be bonded to each other.
A series of the following processes or steps have been taken to manufacture an integrated storage disc (e.g. optical disc) by bonding two substrates (see FIG. 20).
(1) a step of placing a lower disc-shaped substrate D1 on a holding table.
(2) a step of coating an adhesive agent A made of UV cured resin to the lower disc-shaped substrate D1.
(3) a step of placing an upper disc-shaped substrate D2 on the lower disc-shaped substrate D1 to overlay the former on the latter.
(4) a step of rotating the holding table to develop the adhesive agent A interposed between the disc-shaped substrates D1 and D2 on the entire surfaces thereof.
(5) a step of irradiating the developed adhesive agent A made of UV cured resin with UV to cure the adhesive agent A.
The storage disc is manufactured by the aforementioned steps, namely, the two disc-shaped substrates D1 and D2 are bonded to each other to form a single integrated plate.
However in such a bonding method, since the adhesive agent A is developed in a wider range by rotating the holding table, the adhesive agent A sprays outwardly due to by centrifugal force. The periphery of the storage disc is made dirty or soiled by the spray of the adhesive agent A or the adhesive agent A is scattered and lost and hence a significant amount of the adhesive agent A is wasted.
Further, there is a problem that the thickness of the adhesive layer is not at all uniform. Still further, there is another problem that the number of manufacturing steps increases which increases the manufacturing cost because of the necessity of the step of irradiating adhesive agent with UV to cure the adhesive agent.
Accordingly, there is a need for a method of bonding two substrates using an adhesive agent without requiring the developing step of the adhesive agent as set forth above. In the bonding method using an adhesive agent, the adhesive agent is bonded to one of two disc-shaped substrates to be bonded, and thereafter the one disc-shaped substrate is overlaid on the other disc-shaped substrate.
An adhesive agent is generally used in a state where it is bonded to a sheeting, i.e. adhesive sheet body forming a base member, wherein a plurality of adhesive sheets to which the adhesive agent is bonded are bonded to the adhesive sheet body S. Accordingly, it is very ineffective to peel off the adhesive agent on the adhesive sheet one by one manually to bond or transfer it to the lower disc-shaped substrate when two disc-shaped substrates are bonded to each other. Further, bonding operation of an adhesive agent need be automated to incorporate the bonding operation of the adhesive agent to the disc-shaped substrates into the laminating lines of the disc-shaped substrates so as to produce a series of laminating lines which are sequentially controlled as a whole. That is, it is necessary to automatically bond the adhesive agent to the disc-shaped substrate to produce continuous laminating lines.